A quality feature of sensors or measuring instruments is the distribution of errors with which the measured values produced by them are associated: typically, it is required that the measurement errors lie with a specified probability within the specified limits or their mean value and their standard deviation lie within specified limits. In the production of measuring instruments, it may be technically or economically advantageous to ignore the accuracy specification and subsequently to determine the systematic measurement errors consciously accepted thereby by a suitable method—referred to below as calibration—and to reduce said errors computationally or by adjustments of the measuring instrument to such an extent that the accuracy specifications are fulfilled in subsequent measurements.
Calibration of measuring instruments, for example of angle-measuring instruments of the type described in CH 658 514 A5, consists in measuring, with the still uncalibrated instrument, a number of known measurement positions—referred to below as reference positions—and declaring the difference between measured positions and reference positions as measurement errors at the measured points and interpolating these, by a mathematical model describing their dominant components, over the total measuring range of the measuring instrument and processing them numerically and storing them in such a way that they can be computationally compensated in all subsequent measurements, the apparatus correction of the measuring instrument by adjusting devices provided for this purpose representing in principle an alternative. A characteristic feature of the calibration of background art is that it is based on external measurement (for measuring the reference positions).
The use of external systems/methods of measurement for calibrating measuring instruments gives rise to two difficulties, a fundamental one and a technical one. The common cause of both difficulties is the fact that knowledge of the reference positions is also incomplete: incorrect measurements are “corrected” on the basis of other incorrect measurements. This can be effected only by dividing the differences between the two measurements into an instrument error and a reference error. In accordance with background art to date, this division is effected on the basis of a statistical estimation procedure which in turn is based on statistical assumptions relating to the correlation of the errors of the two measurements. The credibility and reliability of these assumptions can be established only by further measurements, with the result that a further calibration problem arises. The basically endless cascade of calibration which begins in this way—the fundamental difficulty mentioned—is ended in practice by ensuring that the accuracy of the knowledge of the reference positions is much higher than the accuracy of measurement required by the calibrated measuring instrument. This gives rise to a technical difficulty that a more accurate measurement procedure has to be provided for the reference positions for each measuring instrument to be calibrated, which, for example in the case of angle measurements with accuracies of angular measurement in the sub-angular second range, is technically complicated and hence uneconomical. Moreover, owing to the technical requirements with respect to the reference positions, calibration methods of background art are generally carried out by the manufacturer, which makes it more difficult to effect continuous calibration of the measuring instrument for compensation of environmental influences and wear and aging processes.
The problems inherent to background art to date can be solved only by eliminating their cause, i.e. effecting the calibration without the use of external measuring systems/methods.